Frequency Synthesizer Circuits; Vco; Pll (Phase Locked Loop) Circuit; Voltage Diagrams - Icom IC-A210 Service Manual

3-3 FREQUENCY SYNTHESIZER CIRCUITS

VCO

A VCO is an oscillator whose oscillating frequency is
determined by the applied volatge. This transceiver has two
VCOs; RX VCO and TX VCO. The RX VCO generates the
1st LO signals for the 1st IF signal produce, and TX VCO
generates TX signal.
The voltage applied to each VCO is passed through the
ripple filter which removes ripples on the VCC line.
• RX VCO
The RX VCO oscillates 79.150 to 98.125 MHz LO signals
for normal RX and WX channels. The generated 1st LO
signals are applied to the 1st mixer (IC68, pin 6) via the
buffer AMPs (Q33, Q35, Q45), TX/RX SW (D36) and the
LPF (harmonic filter).
• TX VCO
The TX VCO oscillates 118.000 to 136.975 MHz transmit
signals. The generated TX signal is applied to the RF
amplifier (Q41) via the buffer AMPs (Q33, Q35), TX/RX SW
(D36).
A portion of the VCO output is applied to the PLL IC via the
buffer and harmonic filter.
• FREQUENCY SYNTHESIZER CIRCUITS

3-4 VOLTAGE DIAGRAMS

Voltage from the power supply is routed to the whole of the transceiver via regulators and switches.
1stLO TX
D36
TX/RX
SW
Q35
BUFF
LPF
L51 56 57
(harmonic
C218 219 225 312
filter)
6
8
X2 Q38
TCXO BUFF
Q29
R8
SW
IC44
5V
+5
REG
IC42
8V +8
REG
IC2 IC16
LINE
10C
+5
+3
FILTER
REG
REG
MAIN UNIT
FRONT UNIT

PLL (Phase Locked Loop) CIRCUIT

The PLL circuit provides stable oscillation for both of the TX
and 1st LO frequencies (for RX). By comparing feedbacked
VCO output and reference frequency signals, the PLL
corrects the difference of the frequencies.
A portion of RX/TX VCO output is applied to the PLL IC via
buffer (Q33) and harmonic filter. The applied VCO output
is divided according to the serial data including divide ratio
from the CPU, at the prescaler and programmable divider.
In the same way, the reference frequency signal from the
TCXO is applied to the PLL IC and divided so that these
two applied signals are the same frequency.
The divided and frequency-matched signals (VCO output
and the reference frequency signals) are applied to the
phase comparator and phase-compared. The resulted
phase difference is detected as a phase-type signal, and
level-adjusted at the charge pump then output. The output
pulse type signal is passed through the loop filter to be
converted into the DC voltage (=Lock Voltage).
Applying the lock voltage to the variable capacitor which
composes a part of the resonator of RX/TX VCO, the
capasitance of variable capacitor changes corresponding
to the appled lock voltage. This causes the change of
resonation frequency that determine the VCO oscilating
frequency to keep the VCO frequency constant.
When the oscillation frequency drifts, its phase changes
from that of the reference frequency, causing a lock voltage
change to compensate for the drift in the VCO oscillating
frequency.
Q5 D21
Q33
BUFF TX VCO
IC47
2
PLL LOOP
RX VCO
IC FILTER
14
R216 218 Q6 D22
C214 215
Q32
Q30
8T1 8T2
SW
SW
PA UNIT
Q11
POWER
SW
10C
+16
16V
REG
IC31 IC75
10 V
REG
3 - 4
Q42
RIPPLE
FIL
8V
+B
1
2
PWR
SW